1. Field of the Invention
The present invention relates to a time division multiplex system having time division multiplexing of pulse-modulated signals comprising pulses of identical shape and width, particularly of optically transmittable pulse phase modulated (PPM) signals.
2. Description of the Prior Art
In such time division multiplex systems, the individual pulse-modulated signals are transmitted in time slots respectively assigned to time channels with the respectively identical chronological position in so-called multiplex frames constantly following one another. In order to guarantee synchronism between devices of the time division multiplex system of the transmission side and on the receiving side, the temporal position of each frame and the temporal position of the individual time slots therein must be clearly defined.
To this end, the beginning of each frame, or of at least each, for example second pulse frame can be defined by means of transmitting a frame identification signal and, proceeding therefrom, one can then derive the temporal position of the subsequent time slots. Such a frame identification signal can be provided, for example, by a frame identification pulse provided at the beginning of the frame, the frame identification pulse being broader than the remaining pulses of the pulse frame and, therefore, being distinguishable from the latter. Such a frame identification, however, uses a corresponding time period of the pulse frame and, moreover, requires a corresponding expense in evaluation.
The expense is even greater when a special pilot tone or a so-called frame identification word is transmitted at the beginning of a frame, the pilot tone or frame identification word then being identified at the receiving side by a corresponding search circuit.